The mood stabilizer lithium inhibits a select group of enzymes, including glycogen synthase kinase-3 (GSK-3). However, it is unclear if lithium?s inhibition of GSK-3 is relevant for its antimanic and antidepressant effectiveness. We are utilizing biochemical, cellular, histochemical, genomic, and behavioral validation approaches to investigate whether the inhibition of GSK-3 is an integral part of the mechanism of lithium?s clinical effects. We are utilizing rodent behavioral models and two distinct but complementary approaches (pharmacologic inhibition and transgenic gene expression) in an attempt to further validate GSK-3 as a possible mediator of lithium?s therapeutic effects. Specifically, one of the primary targets of GSK-3 is the transcription factor beta-catenin. We have shown that lithium administration to rats, in a clinically relevant paradigm, results in an increase in beta- catenin levels. We are studying the effects of both over-expression and under-expression of beta-catenin in the mouse brain as well as pharmacological mechanisms to increase beta-catenin. Using both approaches, we have found the rodents exhibit both antidepressant-like and antimanic-like behavior. Combined, these data support the hypothesis that lithium may exert its antidepressant and antimanic effects through inhibition of GSK-3, and that novel small-molecule GSK-3 inhibitors may represent a truly novel class of medications useful for the treatment of bipolar disorder and depression. Validation of lithium?s therapeutic target will require clinical trials with novel inhibitors, the development of which is in progress.